1. Field of the Invention
This invention relates to thermochemical conversion of hydrocarbon and other organic fuels to produce hydrogen and carbon monoxide. More particularly, this invention relates to method and apparatus for thermochemically converting hydrocarbon and other organic fuels to produce hydrogen and carbon monoxide in which hot exhaust gases from internal combustion engines, furnace exhaust gases and the like are utilized as heating media or reactants in the conversion process. As used herein, the terms “thermochemical conversion” and “thermochemically converting” refer to processes in which the hydrocarbon and/or other organic fuels are reformed or thermally decomposed. This invention also relates to an apparatus for thermochemically converting hydrocarbon and other organic fuels which may be suitable for vehicular installation and use. This invention further relates to a catalytic reactor for thermochemically converting hydrocarbon fuels that performs well at intermediate and low temperatures, in the range of about 500° F. to about 1500° F. This invention further relates to a method of operating an internal combustion engine using a thermochemically converted fuel. In the thermochemical conversion processes of this invention, the thermal efficiency of the combustion process is increased, resulting in lower fuel consumption and related increase in thermal process efficiencies.
2. Description of Related Art
Thermal processes often reject large amounts of heat. The percentage of rejected or waste heat is particularly large in processes in which chemical energy or fuel value is converted into mechanical energy. Exemplary of such processes are engines. Reciprocating internal combustion engines have thermal efficiencies in the range of about 25% to 40% depending upon design and age of the engine. Diesel engines typically have higher efficiencies than gasoline engines. A typical, modern diesel engine may have a mechanical efficiency of about 35%. Thus, depending upon the type of engine employed, up to about 75% of the fuel value consumed by these engines is converted into waste heat. Since the invention of these engines, efforts have been ongoing to increase their mechanical efficiency; and as fuel costs increase, these efforts become more urgent.
One particular approach for utilizing part of the waste heat generated by reciprocating internal combustion engines is thermochemical recuperation or TCR. In this process, a portion of the waste heat is recirculated into the engine. There are at least two approaches to recirculating waste heat into the engine, both of which involve thermochemical fuel reforming.
In the first of these processes, referred to as thermochemical recuperative reforming, a hydrocarbon fuel is mixed with a large amount of steam, resulting in a molar ratio of steam to fuel in the range of about 2 to 3. Using the waste heat recovered from the hot engine exhaust gases, the steam/fuel mixture is catalytically converted into a gas mixture that contains large amounts of hydrogen and carbon monoxide, which may be returned through the fuel intake back to the engine. The reforming reaction can follow different paths based on process conditions and will, accordingly, produce a variety of reaction end products. The basis for thermochemical heat recovery is the application of low temperature, endothermic fuel conversion. When reforming a fuel, thereby producing large percentages of hydrogen and carbon monoxide, the reaction is endothermic; that is, the reaction consumes heat. As a result, sensible heat is consumed in the reforming reaction. The consumed, sensible heat provided by cooling of hot exhaust gases is used for converting fuel into products with different chemical compositions and a higher fuel heating value. Thus, comparatively low-value waste heat is converted into a higher heating value fuel.
In the second of these processes, referred to as exhaust gas reforming (EGR), exhaust gas recirculation, or more descriptively as catalytic exhaust reforming, the water vapor in the engine exhaust gases is used as reactant for the reforming reaction. In yet another application, other organic fuels such as methanol and ethanol are thermally decomposed in a homogeneous or catalytic reaction, preferably in the presence of water vapor.
Common to all these conversion processes is an increase in heating value of the employed fuel. Thermodynamic considerations immediately show that the increase in heating value can be substantial. For steam reforming of methane, a maximum increase of 25.7% can be predicted. Realistically, only a fraction of this potentially available energy can be converted. For the previously mentioned decomposition reactions of methanol and ethanol, the values are much lower and amount to 13.4% and 20.0% respectively. If one conservatively assumes a 30% efficiency for these three fuel conversion processes, one can predict an increase of engine efficiencies for the three different fuels mentioned of 7.5% for steam reforming of methane, and 4% or 6.0% for catalytic and thermal conversion of methanol and ethanol. These figures are not large but are respectable when compared with historic annual engine efficiency improvements.
U.S. Pat. No. 6,508,209 B1 to Collier, Jr. teaches the introduction of natural gas and/or propane into a reforming reactor for the purpose of converting or reforming a portion thereof to hydrogen and carbon monoxide, providing a gaseous mixture exiting the reactor comprising methane and/or propane, hydrogen, steam, nitrogen, carbon monoxide, and carbon dioxide. The gaseous mixture is mixed with air to provide a gaseous fuel mixture and air combination which is introduced into the internal combustion engine and combusted to produce an exhaust gas. A portion of the exhaust gas is recycled and introduced into the reforming reactor for the purpose of reforming a portion of the gaseous fuel to hydrogen and carbon monoxide. In accordance with one embodiment, the exhaust gas is used, without diluting the combustion charge, for preheating the fuel to be reformed as well as the catalyst bed, for purposes of reforming the fuel.
U.S. Pat. No. 6,855,272 B2 to Burlingame et al. teaches a syngas production process and reforming exchanger in which a first portion of hydrocarbon feed mixed with steam and oxidant is passed through an auto-thermal catalytic steam reforming zone to form a first reformed gas of reduced hydrocarbon content, a second portion of the hydrocarbon feed mixed with steam is passed through an endothermic catalytic steam reforming zone to form a second reformed gas of reduced hydrocarbon content, and the first and second portions of reformed gases are mixed, forming a gas mixture which is passed through a heat exchange zone for cooling the gas mixture, thereby, providing heat to the endothermic catalytic steam reforming zone. The endothermic catalytic steam reforming zone and the heat exchange zone are respectively disposed tube side and shell side within a shell-and-tube reforming exchanger, which comprises a plurality of tubes packed with low pressure drop catalyst-bearing monolithic structures.
Over the past several years, fuel cells, which typically use hydrogen (H2) as a fuel, have been receiving a substantial amount of attention due to their almost emission-free operation. The primary exhaust from a fuel cell using hydrogen, as with other systems in which hydrogen is used as a fuel, is water. It will, thus, be apparent that, in addition to efficiency benefits, substantial environmental benefits may be realized from the use of hydrogen as a fuel in other applications as well, such as internal combustion engines, including reciprocating internal combustion engines and gas turbines. In particular, the hydrogen in the fuel extends the lean operating range of an engine and increases the burning velocity, thereby increasing the combustion rate. Thus, the use of hydrogen in internal combustion engines improves the combustion process and results in increased engine efficiencies. This benefit is largely independent of the heating value increase. Therefore, additional performance enhancements can be expected. The combined effects of thermochemical fuel conversion and combustion process improvements will reduce specific fuel consumption, will lower greenhouse gas emissions, and will open the door to increased utilization of biologically derived fuels.
Notwithstanding the apparent attractiveness of using thermochemical fuel conversion for increasing the efficiency of engines and reducing emissions output, the proposed energy recovery and improved thermal efficiency methods have not found widespread use due to a number of technical problems. One of the major problems is a lack of thermochemical fuel conversion systems suitable for use in vehicular applications.